Capacitor



NOV. 20, 1951 SINK 2,575,364

CAPACITOR Filed Oct. 14, 1948 D/STR/BUTED CAPACITY 2. F/LAME/VT 7'0 \GR/D f27 22 c c, GRID IMPEDANCE F/LAMENT HUM-BALANCING FIG. 4.

FIG. 3. f"

I A I I ZD ANCE 48\ i:

INVEN TOR. ROBERT L. SINK A T TORNE Y Patented Nov. 20, 1951 CAPACITOR Robert L. Sink, Altadenap CaliL, assignor to Consol'idatedEngineering Corporation, Pasadena,

Calii'., a corporation, of California Application October 14,1948, Serial No. 54,421

This invention relates to improvements in low level amplifying circuits such as high gain audio amplifiers. In particular, the invention relates to apparatus for reducing to a minimum the hum resulting from the capacity coupling existing between the filament and grid circuits of the am-- plifier tubes.

The problem of equivalent hum noise at the input grid of a high gain amplifier has received a great deal of attention. The elimination of hum n the input stage of such amplifier has been achieved in various ways. One technique has involved the use of a center tap filament transformer.

Heretofore particular attention has been given to the problem of conduction from the heating element to the cathode, grid, etc. within the vacuum tube. This problem has been materially alleviated by the use of very low impedance connections between the cathode circuit and ground. Additional advantage is obtained by actively biasing the heater either positively or negatively with respect to ground by a sufiicient voltage to prevent the polarities between the filament and cathode from ever reversing.

It has also been found desirable to use an adjustable tap potentiometer connected across the terminals of the heater. The tap of the potentiometer is then attached to ground or preferably to a bias source as described above. -'Adjustment of the potentiometer will generally result in a minimum A. C. hum level.

One element that has been generally overlooked is the capacity coupling that exists between the fllament circuit and the grid circuit. This capacity coupling is generally quite small. It can be shown, however, that even this small capacity can induce a relatively large voltage in the grid circuit. By way of example, assume a perfectly center-tapped and grounded filament transformer is supplying heater power to the input tube. A capacity unbalance of one mmf. (micromicrofarad) I with an average capacitance of 2 mmf. from each of the two filament connections to the grid would result in approximately one mv. (millivolt) hum signal on the grid of the tube if the grid impedance is one megohm to ground. I have found that a one mv. signal appearing at the grid of the tube is' quite normal from what appears to be capacitive unbalance from the two filament posts to the grid connection.

The capacitor of the invention will effectively reduce the hum signal appearing at the grid of the input tube by a factor of five or more to one:v The capacitor is therefore valuable whenever it greater.

2 Claims- (Cl. 175-415) is desired to amplify low-level signals of approximately 10 mv. or less, and when the grid impedance is in the order of 0.1 megohm or Thus if the capacity unbalance produces a 1 mv. hum signal an input signal of 10 mv. would have a noise level'only db below the signal level.

In accordance with the invention I provide what may be termed a hum balancing capacitor comprising an insulating body member, a separate conductive soldering lug attached to each end of the body' member and spaced from each other by the body member, and a conductive member mounted on the body between the lugs and adjustable thereon to increase and decrease respectively the capacitance between the conductive member and the two lugs respectively.

The capacitor is connected in the tube circuit by connecting the filament leads to the two lugs relation to the accompanying drawing in which:

Fig. 1 is an elevation of the capacitor partially cut away to show the means of attaching the soldering lugs to the body;

Fig. 1A is a sectional elevation taken on the line lA-IA of Fig. 1;

Fig. 2 is a diagram which illustrates the functioning of the capacitor;

Fig. 3 is a circuit diagram of an amplifier tube showing the incorporation of the capacitor of the invention therein;

Fig. 4 is a sectional elevation of an alternative form of the capacitor; and

Fig. 5 is a plan view of a portion of the apparatus of Fig. 4.

The capacitor shown in elevation in Fig. 1 and in section in Fig. 1A comprises an insulating rod or body H] which may be of any cross sectional shape. The opposite ends of the rod I0 I are tapped to receive screws H and I2 which hold soldering lugs l3 and M respectively to the rod. A conductive strap I6 is mounted on the rod l0 between the lugs l3, M. The ends of the strap are brought together by a bolt l1 and nut 18 to hold it on the body. The position of the strap on the body may be adjusted by loosening the tween the grid circuit and ground to substantially zero voltage. The bridge circuit is shown in the diagram of Fig. 2 where the capacitance that exists between each side of the filament post and grid is represented by the capacitor Cl, C2 and is given the legend distributed capacity filament to grid. The capacitors C3, C4 are representative of the two sides of the hum balancing capacitor shown in Fig. 1 with an arrow 22 indicating the adjustable strap IS. The other two arms of thebridge circuit consist of the secondary winding 23 of the filament transformer 24.

As described with relation to Fig. l the capacitors C3 and C4 are so related that decreasing C3 will result in an increase in capacitance of C4 and conversely decreasing C4 will increase C3. In this manner a simple bridge balancing circuit is obtained which will effectively reduce to zero the induced grid voltage resulting from the capacity coupling between the posts 25, 25 of the filament 21 and the grid. The grid is not shown in the diagram of Fig. 2 but is connected to the grid impedance 28 in the conventional manner.

This balancing technique can be used to balance out signals of the correct phase relationship from whatever source these signals may come.

For best results the capacitance of C3 and C4 should be as small as possible and still result in obtaining a complete balance. If these capacitors are too large, adjustment will become overly sensitive to the point where it may be impossible to observe that such an adjustment can be made. In practice the hum balancing capacitor has been constructed so that the actual value of the capacity is somewhat less than 1 mmf. This serves as an example of practical application but is not intended as a limitation on the capacitance of the capacitor.

A portion of an actual amplifier circuit is shown in Fig. 3 to illustrate the placement of the hum balancing capacitor therein.

The circuit of Fig. 3 includes a vacuum tube 30 having a filament 21, a cathode 3| and a grid 32. The hum balancing capacitor including the capacitors C3, C4 and the adjustable strap I6 is connected across the filament posts 25, 26. The strap I6 is connected to the grid circuit 32 intermediate the grid and the. grid impedance 28. Low voltage A. C. current is supplied to the filament from a source 34 which is representative of a filament transformer or any conventional A. C. filament supply. The other elements of the circuit in Fig. 3 are conventional and description thereof is omitted since they form no part of the present invention.

The simple form of the hum balancing capacitor shown in Fig. 1 is entirely satisfactory although care must be used in adjustment. Since there is no vernier method of adjusting the strap .16, it is necessary to do so by hand. However, each time the strap is touched, the amplifier is overloaded and no observation of the effects of the adjustment can be made until the hand is removed.

An alternative embodiment of the invention is shown in Fig. 4 and has the advantage of being adjustable by means of a screw driver or the like. This embodiment comprises a rod 40 which again may be of any cross sectional shape and which has soldering lugs 4!, 42 afiixed to its opposite ends by screws 43, 44 respectively. The strap IS in the embodiment of Fig. 1 is replaced by conductive cam 46, a portion of the body 60 is cut away to form a fiat face 41 above which the cam 48 is mounted on a post 48 extending through the body. The post 48 is grooved'at 49 to receive a screw driver and is provided with a retaining clip 50 to hold the assembly in the body. The post 48 is rotatable in a conductive sleeve 52 which extends through the body and forms a bearing surface at 52A upon which the cam 55 rotates. The leads to the filament are, as before, connected to the lugs 6|, (l2 and the lead to the grid circuit is amxed to the sleeve 52. The capacity between the cam 66 and the lugs 6|, d2

is adjustable by rotation of the cam 46. This can be accomplished by means of a screw driver and the effects of the adjustment can be ascertained at the same time. In this respect the apparatus of Fig. 4 has an advantage over that of Fig. 1. Otherwise, however, their functioning is identical.

Numerous means of providing for the adjust- .ment of the strap or conductive member between the lugs ll and 42 may be employed. The one shown in Fig. 4 is merely illustrative of the possibility of carrying out such an adjustment without actually touching the conductive member.

I claim:

1. A capacitor comprising an insulating body, separate electrodes extending into the confines of the body in axial alignment with one another and in fixed spacial relationship, terminal connecting means attached to the outer end of each electrode, a transverse bore through the body intermediate the electrodes, a conductive sleeve mounted in the bore, a post projecting through the sleeve and in contact therewith, a cam mounted on one end of the post adjacent the body, the opposite end of the post being slotted to receive a screw driver, retaining means adjacent said opposite ends of the post to prevent Withdrawal of the post from the sleeve and means for connecting an electrical lead to the sleeve.

2. A capacitor comprising an insulating body, separate electrodes extending into the confines of the body in axial alignment with one another and in fixed spacial relationship, terminal connecting means attached to the outer end of each electrode, a transverse bore through the body intermediate the electrodes, a post projecting through the bore and rotatable therein, a conductive disc eccentrically mounted on one end of the post overlying a portion of the body, means for making electrical connection to said disc, and means formed on the opposite end of the post for engagement and rotation thereof.

ROBERT L. SINK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

